1) Field of the Disclosure
The disclosure relates to radomes, and in particular, to radio frequency (RF) radomes used at high temperatures.
2) Description of Related Art
RF (radio frequency) radomes are structures that may be used on high speed aircraft, missiles, supersonic airframes, spacecraft, and other craft. RF radomes are typically used to cover instruments, such as radar devices and antennas, that transmit and receive electromagnetic and RF radiation, in order to protect such devices from environmental conditions and mechanical stresses. RF radomes are constructed to be substantially transparent to RF radiation over broadband or narrowband frequencies. The surfaces of high speed aircraft, missiles, supersonic airframes, spacecraft, and other craft are often subjected to aerodynamic heating, extreme environmental conditions, and significant mechanical stresses and erosion, which can all affect their performance. Such high speed aircraft, missiles, supersonic airframes, and spacecraft require RF radomes with good thermo-mechanical properties that can survive extended high temperature exposures (e.g., above 700 degrees Fahrenheit), severe thermal gradients, and most weather or atmospheric conditions with low-loss, uniform, and stable signal transmission, at a reasonable cost.
Material selection for an RF radome may affect the RF radome thermo-mechanical properties, operating temperature, strength, impact and weather resistance, dielectric loss, signal transmission, and manufacturing tolerances. For example, known RF radomes may be made of polymeric matrix composites (PMCs), ceramic matrix composites (CMCs) and monolithic ceramic materials. As flight speed increases, the typical solution set progresses from PMCs to CMCs and finally to monolithic ceramics. Examples of PMCs include glass/epoxy, quartz/bismaleimide, quartz/cyanate ester, quartz/polyimide, and alumina-boria-silica fibers/polybenzimidazole. Examples of CMCs include quartz/polysiloxane, quartz/polysilazane, and oxide/oxides such as alumina-boria-silica fibers/aluminum silicate. Examples of monolithic ceramic materials include fully dense silicon nitride (Si3N4), in situ reinforced barium aluminum silicate (IRBAS), reaction bonded silicon nitride (RBSN), polycrystalline glass ceramic, fused silica, and gel cast silicon aluminum oxynitride (SiAlON).
In a typical high speed flight profile, severe atmospheric induced drag can result in elevated surface temperatures on an RF radome structure, such as shown in FIG. 9. The aerodynamic heating is typically most severe at a forward tip of the RF radome and may be gradually reduced with increasing distance from the tip. Since RF radomes are typically made of a single material, the aerodynamic heating in a forward sector of an RF radome often drives the material selection to higher temperature capable materials. Such materials, however, are generally more expensive and may be subject to various limitations. For example, radomes made of PMCs have excellent transmission properties, low weight, low manufacturing costs, good uniformity, and excellent fracture resistance. However, such radomes may have reduced thermal properties and reduced erosion resistance in high speed flight. In addition, excessive temperature can cause PMCs to decompose during flight. Such decomposition may lead to surface roughness which can increase drag and aerodynamic heating and increase deterioration in signal transmission.
Radomes made of CMCs are similar to radomes made of PMCs except that radomes made of CMCs have slightly higher temperature capabilities and consequently can be more stable at high temperatures. Some CMCs can be produced with excellent dimensional control and require no surface treatment such as milling, so that such CMCs are more affordable and less expensive than monolithic ceramics. However, radomes made of CMC can be more expensive than radomes made of PMCs. Radomes made of CMCs may have reduced erosion resistance which may result in excessive material or ply loss. CMC radomes can have significant porosity which may result in fluid intrusion into the radome, may outgas during flight, and may have reduced RF transmission properties.
Radomes made of monolithic ceramics typically have higher temperature capabilities and better erosion resistance than radomes made of PMCs or CMCs. However, radomes made of monolithic ceramics can be significantly more expensive to produce than radomes made of PMCs or CMCs. Such radomes made of monolithic ceramics may require machining on green ceramics and/or grinding of fully hardened ceramics to achieve precision dimensional control which can result in increased production costs and lower yields. Moreover, radomes made of monolithic ceramics may have less robust performance from impact shock loads or high internal stresses from large internal temperature gradients. Radomes made of monolithic ceramics typically have higher dielectric and loss properties that reduce the effectiveness of signal transmission compared to radomes made of PMCs or CMCs.
Thus, existing materials may be expensive and may be subject to reduced performance and surviveability under extended high temperature exposures (e.g., above 400 degrees Fahrentheit), severe thermal gradients, and extreme weather or atmospheric conditions. It is believed that known RF radomes do not use thermal barrier coatings to enhance or extend radome performance capabilities.
Accordingly, there is a need for RF radomes and method having enhanced performance in high temperature applications, enhanced all weather flight capability, enhanced thermal environment surviveability, and that provide advantages over known devices and methods.